It is possible to supply electrical power over a wired telecommunications network such as the Ethernet, for example, from power sourcing equipment (PSE) to a powered device (PD) over a communication link. IP telephones, wireless LAN access points, Bluetooth access points, web cameras, digital still and video cameras, computers, tablets, liquid crystal displays, point-of-sale kiosks, network intercom systems, cellular telephones, security systems, gaming systems, etc. are examples of a PD. This technology is known as Power over Ethernet (PoE). PoE is defined in IEEE standard 802.3-2012 which provides the standard for combining transmission/reception of Ethernet packets (e.g., a communication signal) with transmission/reception of DC power over an Ethernet cable. According to the PoE standard, electrical power can either be injected by an endpoint PSE at one end of a link section or by a midspan PSE at a point along the link section in between network interfaces communicatively connected at opposite ends of the link section. A number of PoE implementations are discussed below.
Referring now to FIGS. 1A-1D, example block diagrams of systems for supplying electrical power in a PoE environment are shown. Particularly, in FIGS. 1A-1C, systems for supplying electrical power in variations where communication is enabled on two of the four twisted pairs (e.g., 10BASE-T, 100BASE-TX). In FIGS. 1A and 1B, electrical power is injected by an endpoint PSE, and in FIG. 1C, electrical power is injected by a midpoint PSE.
For example, as shown in FIGS. 1A and 1B, a network element 102 such as a switch, hub, router, gateway, etc., for example, is communicatively connected with a PD 108 through a communication link 120 (i.e., an Ethernet cable). The communication link 120 includes four twisted pairs 122A-122D. The network element 102 includes an integral PSE 104. In FIG. 1A, electrical power is injected by the PSE 104 onto two data-carrying twisted pairs 122A, 122B through center-tapped transformers 106A, 106B and is conducted from center-tapped transformers 112A, 112B for use by a load 110 of the PD 108. In FIG. 1A, the two non-data-carrying twisted pairs 122C, 122D are not used for communication or power transfer. In FIG. 1B, electrical power is injected by the PSE 104 onto the two non-data-carrying twisted pairs 122C, 122D and is conducted for use by the load 110 of the PD 108. In FIG. 1B, the two data-carrying twisted pairs 122A, 122B are not used for power transfer.
Referring now to FIG. 1C, an example block diagram of a system for supplying electrical power by a midspan PSE is shown. Similarly to FIGS. 1A and 1B, the network element 102 is communicatively connected with the PD 108 through the communication link 120. However, unlike FIGS. 1A and 1B, the network element 102 does not include an integral PSE 104. Instead, a midspan device 104A passes communication signals on the two data-carrying twisted pairs 122A, 122B, and the PSE 104 injects electrical power onto the two non-data-carrying twisted pairs 122C, 122D. The electrical power is conducted for use by the load 110 of the PD 108.
Referring now to FIG. 1D, an example block diagram of a system for supplying electrical power in variations where communication is enabled on four twisted pairs (e.g., 1000BASE-T, 10GBASE-T). Unlike FIGS. 1A-1C, twisted pairs 122A-122D are all data-carrying twisted pairs. It should be understood that electrical power can be supplied over two or four twisted pairs. For example, in some variations, electrical power can be injected by the PSE 104 onto two data-carrying twisted pairs 122A, 122B through center-tapped transformers 106A, 106B and conducted from center-tapped transformers 112A, 112B for use by the load 110 of the PD 108. Alternatively, electrical power can be injected by the PSE 104 onto two data-carrying twisted pairs 122C, 122D through center-tapped transformers 106C, 106D and conducted from center-tapped transformers 112C, 112D for use by the load 110 of the PD 108. It should also be understood that electrical power can be injected by the PSE 104 simultaneously onto two data-carrying twisted pairs 122A, 122B and two data-carrying twisted pairs 122C, 122D, respectively, to supply electrical power to two different loads. In addition, it should also be understood that electrical power can be injected by the PSE 104 simultaneously onto two data-carrying twisted pairs 122A, 122B and two data-carrying twisted pairs 122C, 122D, respectively, to supply electrical power to the load 110 of the PD 108. One of ordinary skill in the art would understand that the load 110 can accept power from a pair of diode bridge circuits such as full wave diode bridge type rectifier circuits depending on whether electrical power is supplied over data-carrying twisted pairs 122A, 122B, data-carrying twisted pairs 122C, 122D or data-carrying twisted pairs 122A, 122B and 122C, 122D. The diode bridge circuits can be part of a power reception circuit, for example.
Line coupling transformers (e.g., transformers 106A-106D and 112A-112D in FIGS. 1A-1D) in an Ethernet interface such as the network element 102 in FIGS. 1A-1D can act as high pass filters. The effect of a high pass filter is often referred to as “droop” because a steady voltage supplied to/received at a line coupling transformer is seen to droop according to the severity of the high pass filter. For example, differences in current flowing through each of the conductors in a twisted pair such as one of twisted pairs 122A-122D in FIGS. 1A-1D, for example, can cause saturation in a center-tapped magnetic device such as a line coupling transformer. If non-equal currents flow toward the center tap of the transformer from either side, the transformer core sees a non-zero current. A sufficiently larger non-zero current degrades the transformer characteristics, which causes droop. This problem can be more pronounced in PoE systems due to the large currents flowing through the transformer, which cause the magnetic core to become saturated and lower the effective inductance. Additionally, even if equal currents (i.e., electrically balanced currents) flow through each conductor of the twisted pair, imperfections during the transformer manufacturing process such as imperfections in the transformer windings, for example, can cause droop. As a result, communication signals that pass through the transformer are distorted.
In addition to current imbalances and manufacturing imperfections, the reduction in open circuit inductance (OCL) of line coupling transformers results in increasing droop. This increase in droop is present independent of the increase caused by current imbalances and/or manufacturing imperfections. While the specified minimum OCL should have theoretically increased to deal with current imbalances in PoE systems, the specified minimum OCL has decreased. For example, the standards for 100BASE-TX Ethernet specified an effective minimum OCL of 350 μH, and many legacy devices complying with the 100BASE-TX Ethernet standards were designed with OCLs up to 700 μH. However, the industry has recently lowered the effective minimum OCL specification for 10/100/1000BASE-T Ethernet to 120 μH. As a result, an Ethernet link can have a transformer with an OCL of 120 μH on one side and a transformer with an OCL of up to 700 μH on the other side. Further, the standards for 10GBASE-T Ethernet specify an effective minimum OCL of 100 μH (and an effective maximum OCL of 160 μH). Additionally, devices complying with the 10GBASE-T Ethernet standards can be designed to be backward compatible with 100/1000BASE-T Ethernet standards using the same transformer. Therefore, there is a risk of Ethernet link degradation due to the lowering of the effective minimum OCL specification.
Referring now to FIG. 2, a graph illustrating example results of droop tests performed on a plurality of Ethernet links is shown. The droop tests were performed according to 1000BASE-T Ethernet standards using standard measurement bench equipment and fixtures. Standard tests have traditionally proven that when the amount of droop is within acceptable limits (i.e., a specified range), a receiver can be designed to recover the communication signal without error. However, FIG. 2 illustrates degradation of Ethernet links caused by lowering of the effective minimum OCL specification. Measured response curve 202 illustrates a distorted communication signal on an Ethernet link between link partners having transformers with OCLs of greater than 350 μH. The amount of droop in curve 202 is within the specified range, and therefore, a receiver is expected to recover the communication signal without error. Measured response curve 204 illustrates a distorted communication signal on an Ethernet link between link partners having transformers with OCLs of greater than 350 μH and 140 μH, respectively. The amount of droop in curve 204 is at the margin of the specified range, and therefore, a receiver may not be capable of recovering the communication signal without error. Measured response curve 206 illustrates a distorted communication signal on an Ethernet link between link partners having transformers with OCLs of 140 μH. The amount of droop in curve 206 is below the specified range, and therefore, a receiver is not expected to recover the communication signal without error.
In some cases, particularly for higher speed Ethernet interfaces such as 1G and 10G links, the link undergoes a training sequence that allows the receiver to adapt the characteristics of the link, which includes compensating for the droop caused by the transformers, by modifying adaptive filters, for example. However, as system parameters change, the adaptive filters modified during the training sequence can become obsolete. For example, as discussed above, transformer characteristics change as the amount of current flowing through the transformer increases or decreases. This becomes particularly important in energy efficient PoE systems where the amount of PoE current can rapidly change over short periods of time.